In a manufacturing process flow of a semiconductor device such as a NAND-type nonvolatile semiconductor storage device, a slimming process employing wet etching may be carried out to form fine patterns. The slimming process may require the etching to progress at a low etch rate and with high precision in order to form fine patterns. Thus, the etch rate is controlled so as not to vary significantly during wafer processing. It is however, difficult to control the etch rate with ease and at low cost when etching at low etch rates.